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The main goal of the "travel alert" is to remind Americans traveling overseas to be cautious. It falls one step below an official U.S. "travel warning," which would advise American citizens against certain foreign travel altogether. With the latest travel alert, the U.S. government isn't "urging Americans not to travel to Europe, just to show vigilance" because of the heightened security concerns there, a senior U.S. official said.

State Department Undersecretary Patrick Kennedy called the situation "a relatively short-term condition that poses a serious risk." The State Department opted against issuing a travel warning because the information available didn't warrant it, he said.

He added that Americans traveling to Europe should continue with their plans, but take precautions. For example, they shouldn't use luggage tags that directly identify them as American, he said. Americans should register their European travel plans with the consular section of the U.S. Embassy via its website (travelregistration.state.gov), so if they run into a dangerous situation, the State Department can more easily locate them. They also should make sure they know how to use local pay phones to alert authorities of any untoward activities they might observe.

Mr. Kennedy also advised Americans "to use common sense," such as moving away from unattended packages, avoiding public demonstrations or civil disturbances and refraining from discussing travel plans with others.

Broad travel alerts are more common for incidents related to weather or health, for example when a volcanic ash cloud hung over Europe earlier this year.

Travelers can get updates on security conditions by calling 202-501-4444 from outside the U.S.